1. Origin of the Invention
The present invention is a continuation-in-part application of pending PCT International Application No. WO91/03143 (PCT/US90/04660), filed Aug. 17, 1990, designating the United States. The Chapter II Demand was timely filed on Mar. 15, 1991, also designating the United States. This pending PCT International patent application is incorporated herein by reference in its entirety.
2. Field of the Invention
The present invention discloses a number of ion emitter tip materials, e.g., filaments or needles, which are used to generate gaseous ions, but which concurrently generate undesirable particles of size of 0.5 microns or less. Thin coatings of silicon on the tips are also described. Specifically, these tip materials and coatings; may be used to maintain Class 1 clean room particle conditions usually associated with the manufacture of electronic devices, especially semiconductors.
2. Description of the Related Art
Semiconductor manufacturers and others need to go to great lengths to maintain a clean processing area, and to prevent particle contamination of critical wafer surfaces. Once a particle is airborne, it becomes a potential contaminant whether it comes from a moving machine or from a surface. In either case, it is prudent to eliminate or decrease the source of the particles.
When the particle source cannot be eliminated, steps need to be taken to reduce the deposition of airborne particles on surfaces. One method is to use bipolar air ionization to reduce surfaces on products.
Present reports concerning particle generation by ionizers show a number of problems. Some results are based on accelerated testing at corona currants of up to 50 times normal operating levels. Some tests used emitter materials that ionizer manufacturers do not use because these materials erode rapidly. The air quality in clean rooms is generally classified according to specific standard criteria, relating the class designation to the number of particles per cubic foot of air at a size of about 0.5 microns. Thus Class 1 conditions refer to fewer than 1 particle of 0.5 micron size per cubic foot of air.
Presently Class 1 cleanroom conditions (i.e., 10 particles of 0.5 microns or larger per cubic foot) are achieved using conventionally available emitter materials, e.g. tungsten-2% thorium. In some applications, Class 10 conditions are not clean enough to provide a satisfactory manufacturing environment. Class 1 conditions are needed. Unfortunately, there is presently no way to predict a priori which ion emitter tip materials can be used to produce Class 1 conditions.
West German patent application DE 36 03647 1A describes the use of a number of materials, metals and alloys, as ion emitter tips. Comparative experiments were performed for 1,000 hours at a 10-fold electrical point load. This patent does not disclose the size or amount of particles emitted using normal electrical work load conditions. The patent does not disclose emitter tip materials which are useful to achieve Class 1 conditions.
R. F. Cheney, et el. in U.S. Pat. No. 3,745,000 described a process for producing tungsten-alloy type electrodes. The tungsten is alloyed with from 0.2 to about 7.0 percent by weight of a Group VIII metal additive which lowers the sintering temperature of tungsten at least about 100.degree. C. A tungsten lead is also described consisting essentially of tungsten and from about 1 to 30 percent by weight of rhenium. The patent does not disclose alloy compositions for ion emitter tip materials which are useful to achieve Class 1 conditions.
R. B. Donovan, et al., (May, 1986) Microcontamination, p. 38, B. Y. Liu, et el. (1985) "Characterization of Electronic ionizers in the Clean Room," 31st Meeting, institute of Environmental Sciences, Las Vegas, Nev., disclose that ionizer particles emitted typically have a mean count diameter of about 0.03 microns. These particle measurements are obtained with a condensation nucleus counter (CNC) and indicate a qualitative difference in ion particle production based on various emitter tip materials. These two references do not disclose specific ion emitter tip materials useful to achieve Class 1 conditions.
U.S. Patents of general background interest in the ion emitter for the reduction of airborne particle contamination in a clean room includes J. Sachetano, 4,902,640; A. J. Steinman et al., 4,901,194; H. Ooga, et al., 4,725,874; 4,894,253; A. Kawakatsu, 4,873,200; R. W. Barr, 4,739,214; and W. R. Heineman et al. 4,894,253.
All articles, patents, references and standards cited are incorporated herein by reference in their entirety.
It is therefore apparent from the above that a need exists to identify emitter tip materials that would be useful for generating gaseous ions in a manner compatible with Class I particle conditions in clean rooms. The present invention provides a solution to this need, by the use of specific metals and metal alloys as the ion emitter tips and coatings on the emitter tips.